Integrated circuits are being developed to provide a wide range of functions in telephone applications. One application of such integrated circuits is to store one or more telephone numbers and recall the selected number for transmission over the telephone line in place of the user manually entering the number. But with the types of integrated circuits most applicable for telephone functions the stored number can be lost if the power supply to the integrated circuit should drop to a low level. The power for the integrated circuit is typically not provided locally but is received over telephone lines from a central office. The power voltages received at the telephones are subject to wide variations in amplitude and occasionally there is a total loss of the power voltage.
If a number should be stored in the integrated circuit memory and then lost due to a voltage reduction or transient a meaningless number will be introduced into the memory. This would not be the number which was previously stored. Thus, there exists a need for a circuit which can detect the loss of voltage supplied to the integrated circuit such that the circuit can be cognizant of the validity of the number stored in memory and therefore will not read an erroneous number from memory.